Among semiconductor laser elements, nitride semiconductor laser elements have been used as light sources for reproducing and recording operations of optical disc mediums represented by Blu-ray discs and the like. It has also been examined to adjust wavelengths of emission light to use nitride semiconductor laser elements as light sources for processing such as laser annealing or light sources for displays of projectors, laser television sets, and the like. Furthermore, nitride semiconductor laser elements have been examined to be combined with phosphors or the like to be solid-state light sources for displays, lighting devices, and the like.
For general semiconductor lasers including nitride semiconductor laser elements, it has been known that some phenomena prevent oscillation of the semiconductor lasers. For example, increase of a light output gradually increases an operation current under certain light-output conditions. This is a deterioration phenomenon which prevents laser oscillation. The increase of light output further causes a phenomenon called optical breakdown which also prevents oscillation of semiconductor lasers. In order to gain a high light output from nitride semiconductor laser elements, various techniques have been developed to suppress or prevent such deterioration of nitride semiconductor laser elements.
Examples of such conventional techniques are disclosed in PTLs (Patent Literatures) 1 and 2. These patent literatures disclose structures capable of suppressing a sudden death that causes optical breakdown. It is thereby possible to increase a light output up to several hundreds of mW without the deterioration.
More specifically, according to the patent literatures, conventional semiconductor laser elements have a pair of facing end faces (a light-emitting end face is referred to as a front end face, and the other end face is referred to as a rear end face) which are covered with protection films. The protection film covering the light-emitting end face, from which laser light is outputted, is manufactured robust and stable to suppress optical breakdown.
Conventional nitride semiconductor laser element 400 will be described with reference to FIG. 18. Conventional nitride semiconductor laser element 400 has nitride semiconductor layer 410 including an active layer (light-emitting layer) on an n-type GaN substrate. Nitride semiconductor layer 410 has front end face 413 and rear end face 414. On front end face 413, there is provided an end face coat film that includes first end face coat film 415 and second end face coat film 416. On rear end face 414, there is provided end face coat film 417. It is disclosed that first end face coat film 415 is an aluminum nitride (AlN) film, and that second end face coat film 416 is an aluminum oxide (Al2O3) film.
In the above-described conventional structure, the AlN film serving as the first end face coat film is a hexagonal crystalline film. In comparison to general AlN films, a hexagonal crystalline film further prevents oxidation and peeling of the end face of the nitride semiconductor laser element. Optical breakdown and end face deterioration are thereby suppressed. As a result, a light output is increased up to several hundreds of mW.